Estimating the operating point on a non-linear traveling wave tube amplifier

ABSTRACT

A method, apparatus, article of manufacture, and a memory structure provide the ability to determine an input operating point and an output operating point on a non-linear traveling wave tube amplifier (TWTA). The non-linearity of the TWTA is measured. An input roots mean-square (RMS) value of an input signal used to measure the non-linearity of the TWTA is computed. The RMS value identifies an input operating point of the measured non-linearity of the TWTA. Lastly, an output operating point is obtained.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119(e) of thefollowing U.S. Provisional Patent Applications, which are incorporatedby reference herein:

U.S. Provisional Patent Application No. 60/421,289, filed Oct. 25, 2002by Ernest C. Chen and Shamik Maitra, entitled “ESTIMATING THE OPERATINGPOINT ON A NONLINEAR TRAVELING WAVE TUBE AMPLIFIER”; and

U.S. Provisional Patent Application No. 60/510,368, filed on Oct. 10,2003, by Ernest C. Chen, entitled “IMPROVING TWTA AM-AM AND AM-PMMEASUREMENT”.

This is a continuation-in-part application and claims the benefit under35 U.S.C. §120 of the following co-pending and commonly-assigned U.S.utility patent applications, which are incorporated by reference herein:

Utility application Ser. No. 09/844,401, filed Apr. 27, 2001, by ErnestC. Chen, entitled “LAYERED MODULATION FOR DIGITAL SIGNALS;” and

U.S. application Ser. No. 10/165,710, filed on Jun. 7, 2002, by ErnestC. Chen, entitled “SATELLITE TWTA ON-LINE NON-LINEARITY MEASUREMENT.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to systems and methods for transmittingdata, and in particular to a system and method for estimating atraveling wave tube amplifier operating point to accurately reproducetransmitted signals.

2. Description of the Related Art

Digital signal communication systems have been used in various fields,including digital TV signal transmission, either terrestrial orsatellite. As the various digital signal communication systems andservices evolve, there is a burgeoning demand for increased datathroughput and added services. However, it is more difficult toimplement either improvement in old systems or new services when it isnecessary to replace existing legacy hardware, such as transmitters andreceivers. New systems and services are advantaged when they can utilizeexisting legacy hardware. In the realm of wireless communications, thisprinciple is further highlighted by the limited availability ofelectromagnetic spectrum. Thus, it is not possible (or at least notpractical) to merely transmit enhanced or additional data at a newfrequency.

The conventional method of increasing spectral capacity is to move to ahigher-order modulation, such as from quadrature phase shift keying(QPSK) to eight phase shift keying (8PSK) or sixteen quadratureamplitude modulation (16QAM). Unfortunately, QPSK receivers cannotdemodulate conventional 8PSK or 16QAM signals. As a result, legacycustomers with QPSK receivers must upgrade their receivers in order tocontinue to receive any signals transmitted with an 8PSK or 16QAMmodulation.

It is advantageous for systems and methods of transmitting signals toaccommodate enhanced and increased data throughput without requiringadditional frequency. In addition, it is advantageous for enhanced andincreased throughput signals for new receivers to be backwardscompatible with legacy receivers. There is further an advantage forsystems and methods which allow transmission signals to be upgraded froma source separate from the legacy transmitter.

It has been proposed that a layered modulation signal, transmittingnon-coherently both upper and lower layer signals, can be employed tomeet these needs. Such layered modulation systems allow higherinformation throughput with backwards compatibility. However, even whenbackward compatibility is not required (such as with an entirely newsystem), layered modulation can still be advantageous because itrequires a traveling wave tube amplifier (TWTA) peak power significantlylower than that for a conventional 8PSK or 16QAM modulation format for agiven throughput.

To provide a layered modulation scheme (as described in detail below), areconstructed upper layer signal is subtracted from a received compositesignal to reveal a lower layer signal. As such, the lower-layer signalperformance is impacted by how closely the upper-layer signal can bereconstructed relative to the original signal. In other words, the lowerlayer signal performance is impacted by the fidelity of thereconstructed signal. Thus, layered modulation requires cleancancellation of the upper-layer signal to expose the lower-layer signalfor further processing. Clean cancellation requires TWTAnon-linearity/distortion to be accurately reproduced in thereconstruction of the upper-layer signal. Accurate reproduction of TWTAnon-linearity in turn requires knowledge about the TWTA operating point.However, such an accurate reproduction and knowledge of the operatingpoint presents a significant roadblock.

With a TWTA, there is a region of approximate linearity, in which theoutput power is nearly proportional to the input power, followed by acurved transition to a point where the output power levels off andreaches a maximum. At this point (i.e., when the TWTA curve becomesnon-linear), the amplifier is said to have reached saturation. Due tothis non-linearity and to avoid intermodulation, the input power isoften “backed off” by a particular amount (e.g., 6 dB). The resultingpoint on the curve after the input power is “backed off” is referred toas the operating point of the TWTA. When subsequently reconstructing theupper layer signal, the amount of distortion/non-linearity used tocreate the original signal serves to increase the fidelity of thereconstructed signal. Thus, to produce a high fidelity reconstructedupper layer signal, knowledge of the non-linearity as well as theoperating point is useful. Accordingly, the inclusion of (or taking intoaccount) TWTA non-linearity (and operating point) may improveupper-layer signal cancellation ratio by 10 dB or more (i.e., the ratiobetween non-linearity-induced noise before and after cancellation isimproved).

Errors in the estimation of the operating point can have a significantimpact when reconstructing the upper layer-signal. The impact ofamplitude (AM-AM [amplitude modulation to amplitude modulation]) andphase (AM-PM [amplitude modulation to phase modulation]) operating pointerrors may be individually analyzed based on shift analysis. Individualimpacts may then be combined for total impact. To evaluate performanceimpacts, the synthesis of a layer-modulated signal with known TWTAnon-linearity and system/representative operating CNR (carrier to noiseratio) may be used. The upper-layer cancellation error may then becalculated for each amount of simulated operating point error in thesignal reconstruction process. Thus, the upper layer cancellation ratiomay be plotted against the operating point displacement. Thecancellation error can then be converted into an amount of lower-layerCNR degradation, which increases the CNR required for signals of bothupper and lower layers. Such an increased CNR illustrates thesignificance of operating point estimation errors.

FIGS. 16A and 16B illustrate the impact of operating point errors insignal reconstruction. In FIGS. 16A and 16B, the sensitivity of signalreconstruction error is plotted against the TWTA input operating pointerror. The effective noise is calculated as a measure of signalreconstruction error.

In FIG. 16A, a set of generic TWTA non-linearity curves are used. Thesignal reconstruction process is assumed to have full knowledge aboutthe non-linearity curves but is otherwise uncertain about the operatingpoint. The performance plots of FIG. 16A indicate that cancellationerrors are below −25 dB for an input operating point error up to about+/−1 dB.

In FIG. 16B, the performance plots are based on the same TWTAnon-linearity but with an input backoff of 8 dB. With such an inputbackoff, there is improved linearity, that is less susceptible to TWTAoperating point error. As a result, reconstruction and cancellationerrors are greatly reduced as indicated in FIG. 16B. The effective noiseis below −33 dB with an input operating error up to about +/−1 dB.

Accordingly, there is a need for systems and methods for implementinglayered modulation systems that accurately determine TWTA non-linearityand the operating point.

In the prior art, the TWTA operating point is obtained from telemetrytracking and control (TT&C) commands that set the operating point of theTWTA (assuming that TWTA characteristics have little changed since thesatellite was launched). In other words, the operating point set by TT&Ccommands during pre-launch measurements is used post-launch afterreceiving the signals from the satellite. However, TWTA characteristicsincluding the non-linearity and operating point may change over time(including after satellite launch).

Accordingly, what is needed is a system and method for accuratelydetermining the non-linearity and operating point of a TWTA as itchanges over time. The present invention meets this need and providesfurther advantages as detailed hereafter.

SUMMARY OF THE INVENTION

To address the requirements described above, the present inventiondiscloses a method and apparatus for measuring and applying thenon-linearity of a traveling wave tube amplifier, such as in satellitecommunications involving layered modulation. estimating the operatingpoint on a non-linear traveling wave tube amplifier (TWTA). In thisregard, the invention aids in the accurate extraction of a lower-layersignal in a layered modulation scheme. Such an accurate extractionminimizes the amounts of power required for both layers of a signal andalso helps to monitor the health of a TWTA.

To measure/apply the non-linearity of the TWTA, the operating points(input and/or output) for the TWTA are also determined. Initially, thenon-linearity of the TWTA is measured (e.g., using a measuring module).For example, the TWTA non-linearity may be measured at a local receiver,or at a broadcast center (in which case, the non-linearity is downloadedto a local receiver [e.g., for layered modulation and otherapplications]). As part of such a non-linearity measurement, variousinput and output values/points are processed to create the non-linearitycurve. An input root-mean-square (RMS) value of the input signals usedto measure the non-linearity is computed. The RMS value identifies aninput operating point of the measured non-linearity of the TWTA. Inaddition, an output operating point may also be obtained (e.g., by ameasuring module). The output operating point may be based on an RMSvalue of the various output values/points used in measuring the TWTAnon-linearity. Alternatively, the output operating point may simply bebased on the corresponding point (to the input RMS value) on the TWTAnon-linearity curve.

Once the non-linearity has been measured and operating pointsobtained/computed, an upper layer signal (as part of the layeredmodulation scheme) may be reconstructed (e.g., by the receiver). Such areconstruction is more accurate since the appropriate levels ofdistortion accountable to the TWTA non-linearity are accounted for. Inaddition, the measured non-linearity may be offset to simplify thereconstruction of the upper signal. Such an offsetting may provide forscaling an input amplitude value and output amplitude value of themeasured non-linearity to place the output operating point at a desiredpoint. Such a scaling may be conducted by subtracting a measured inputoperating point value from all input values in a log domain.Accordingly, the scaling may also be conducted by subtracting a measuredoutput operating point value from all operating values in the logdomain. The scaling may also be conducted by subtracting a measuredphase value at the output operating point from phase values of alloutput points used to measure the non-linearity of the TWTA.

In addition to the above, when offsetting the measured non-linearity,certain data may fall outside of the measured non-linearity. To accountfor such data, bounding points may be placed beyond the end points (thatare used to measure the non-linearity). Such bounding points may then beused to interpolate data. Further, the input operating point and outputoperating point may also be mapped to a particular level to avoidfractional overflow.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 is a diagram illustrating an overview of a single satellite videodistribution system;

FIG. 2 is a block diagram showing a typical uplink configuration for asingle satellite transponder;

FIG. 3A is a diagram of a representative data stream;

FIG. 3B is a diagram of a representative data packet;

FIG. 4 is a block diagram showing one embodiment of the modulator forthe uplink signal;

FIG. 5 is a block diagram of an integrated receiver/decoder;

FIGS. 6A–6C are diagrams illustrating the basic relationship of signallayers in a layered modulation transmission;

FIGS. 7A–7C are diagrams illustrating a signal constellation of a secondtransmission layer over the first transmission layer after first layerdemodulation;

FIG. 8A is a diagram showing a system for transmitting and receivinglayered modulation signals;

FIG. 8B is a diagram showing an exemplary satellite transponder forreceiving and transmitting layered modulation signals;

FIG. 9 is a block diagram depicting one embodiment of an enhanced IRDcapable of receiving layered modulation signals;

FIG. 10A is a block diagram of one embodiment of the enhancedtuner/modulator and FEC encoder;

FIG. 10B depicts another embodiment of the enhanced tuner/modulatorwherein layer subtraction is performed on the received layered signal;

FIGS. 11A and 11B depict the relative power levels of exampleembodiments of the present invention;

FIG. 12 illustrates an exemplary computer system that could be used toimplement selected modules or functions the present invention;

FIG. 13 is a flow chart illustrating the determination of the operatingpoint in accordance with one or more embodiments of the invention;

FIGS. 14A and 14B are block diagrams of a basic system for measuringperformance maps in accordance with one or more embodiments of theinvention;

FIG. 14C is a flowchart illustrating a method for measuring performancemaps in accordance with one or more embodiments of the invention;

FIG. 15A illustrates an intuitive algorithm for obtaining the outputin-phase and quadrature components in accordance with one or moreembodiments of the invention;

FIG. 15 B illustrates a computationally efficient algorithm forobtaining the input and output operating points in accordance with oneor more embodiments of the invention; and

FIGS. 16A and 16B illustrate the impact of operating point errors insignal reconstruction.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following description, reference is made to the accompanyingdrawings which form a part hereof, and which is shown, by way ofillustration, several embodiments of the present invention. It isunderstood that other embodiments may be utilized and structural changesmay be made without departing from the scope of the present invention.

1. Overview

The invention provides a method of determining/estimating the operatingpoint of a TWTA. The operating point is estimated at the same time TWTAnon-linearity is measured. Therefore, no extra measurement proceduresare required for the determination of the operating point, and themeasured non-linearity is always up-to-date, allowing the measurement tofollow any changes in TWTA characteristics over time.

2. Video Distribution System

FIG. 1 is a diagram illustrating an overview of a single satellite videodistribution system 100. The video distribution system 100 comprises acontrol center 102 in communication with an uplink center 104 via aground or other link 114 and with a subscriber receiver station 110 viaa public switched telephone network PSTN) or other link 120. The controlcenter 102 provides program material (e.g. video programs, audioprograms and data) to the uplink center 104 and coordinates with thesubscriber receiver stations 110 to offer, for example, pay-per-view(PPV) program services, including billing and associated decryption ofvideo programs.

The uplink center 104 receives program material and program controlinformation from the control center 102, and using an uplink antenna 106and transmitter 105, transmits the program material and program controlinformation to the satellite 108 via uplink signal 116. The satellitereceives and processes this information, and transmits the videoprograms and control information to the subscriber receiver station 110via downlink signal 118 using transmitter 107. The subscriber receivingstation 110 receives this information using the outdoor unit (ODU) 112,which includes a subscriber antenna and a low noise block converter(LNB).

In one embodiment, the subscriber receiving station antenna is an18-inch slightly oval-shaped Ku-band antenna. The slight oval shape isdue to the 22.5 degree offset feed of the LNB (low noise blockconverter) which is used to receive signals reflected from thesubscriber antenna. The offset feed positions the LNB out of the way soit does not block any surface area of the antenna minimizing attenuationof the incoming microwave signal.

The video distribution system 100 can comprise a plurality of satellites108 in order to provide wider terrestrial coverage, to provideadditional channels, or to provide additional bandwidth per channel. Inone embodiment of the invention, each satellite comprises 16transponders to receive and transmit program material and other controldata from the uplink center 104 and provide it to the subscriberreceiving stations 110. Using data compression and multiplexingtechniques the channel capabilities, two satellites 108 working togethercan receive and broadcast over 150 conventional (non-HDTV) audio andvideo channels via 32 transponders.

While the invention disclosed herein will be described with reference toa satellite-based video distribution system 100, the present inventionmay also be practiced with terrestrial-based transmission of programinformation, whether by broadcasting means, cable, or other means.Further, the different functions collectively allocated among thecontrol center 102 and the uplink center 104 as described above can bereallocated as desired without departing from the intended scope of thepresent invention.

Although the foregoing has been described with respect to an embodimentin which the program material delivered to the subscriber 122 is video(and audio) program material such as a movie, the foregoing method canbe used to deliver program material comprising purely audio informationor other data as well.

2.1 Uplink Configuration

FIG. 2 is a block diagram showing a typical uplink configuration for asingle satellite 108 transponder, showing how video program material isuplinked to the satellite 108 by the control center 102 and the uplinkcenter 104. FIG. 2 shows three video channels (which may be augmentedrespectively with one or more audio channels for high fidelity music,soundtrack information, or a secondary audio program for transmittingforeign languages), a data channel from a program guide subsystem 206and computer data information from a computer data source 208.

The video channels are provided by a program source 200A–200C of videomaterial (collectively referred to hereinafter as program source(s)200). The data from each program source 200 is provided to an encoder202A–202C (collectively referred to hereinafter as encoder(s) 202). Eachof the encoders accepts a program time stamp (PTS) from the controller216. The PTS is a wrap-around binary time stamp that is used to assurethat the video information is properly synchronized with the audioinformation after encoding and decoding. A PTS time stamp is sent witheach I-frame of the MPEG encoded data.

In one embodiment of the present invention, each encoder 202 is a secondgeneration Motion Picture Experts Group (MPEG-2) encoder, but otherdecoders implementing other coding techniques can be used as well. Thedata channel can be subjected to a similar compression scheme by anencoder (not shown), but such compression is usually either unnecessary,or performed by computer programs in the computer data source (forexample, photographic data is typically compressed into *.TIF files or*.JPG files before transmission). After encoding by the encoders 202,the signals are converted into data packets by a packetizer 204A–204F(collectively referred to hereinafter as packetizer(s) 204) associatedwith each program source 200.

The data packets are assembled using a reference from the system clock214 (SCR), and from the conditional access manager 210, which providesthe service channel identifier (SCID) to the packetizers 204 for use ingenerating the data packets. These data packets are then multiplexedinto serial data and transmitted.

2.2 Broadcast Data Stream Format and Protocol

FIG. 3A is a diagram of a representative data stream. The first packetsegment 302 comprises information from video channel 1 (data comingfrom, for example, the first video program source 200A). The next packetsegment 304 comprises computer data information that was obtained, forexample from the computer data source 208. The next packet segment 306comprises information from video channel 5 (from one of the videoprogram sources 200). The next packet segment 308 comprises programguide information such as the information provided by the program guidesubsystem 206. As shown in FIG. 3A, null packets 310 created by the nullpacket module 212 may be inserted into the data stream as desiredfollowed by further data packets 312, 314, 316 from the program sources200.

The data stream therefore comprises a series of packets (302–316) fromany one of the data sources (e.g. program sources 200, program guidesubsystem 206, computer data source 208) in an order determined by thecontroller 216. The data stream is encrypted by the encryption module218, modulated by the modulator 220 (typically using a QPSK modulationscheme), and provided to the transmitter 105/222, which broadcasts themodulated data stream on a frequency bandwidth to the satellite via theantenna 106. The receiver 500 at the receiver station 110 receives thesesignals, and using the SCID, reassembles the packets to regenerate theprogram material for each of the channels.

FIG. 3B is a diagram of a data packet. Each data packet (e.g. 302–316)is 147 bytes long, and comprises a number of packet segments. The firstpacket segment 320 comprises two bytes of information containing theSCID and flags. The SCID is a unique 12-bit number that uniquelyidentifies the data packet's data channel. The flags include 4 bits thatare used to control other features. The second packet segment 322 ismade up of a 4-bit packet type indicator and a 4-bit continuity counter.The packet type identifies the packet as one of the four data types(video, audio, data, or null). When combined with the SCID, the packettype determines how the data packet will be used. The continuity counterincrements once for each packet type and SCID. The next packet segment324 comprises 127 bytes of payload data, which in the cases of packets302 or 306 is a portion of the video program provided by the videoprogram source 200. The final packet segment 326 is data required toperform forward error correction.

FIG. 4 is a block diagram showing one embodiment of the modulator 220.The modulator 220 optionally comprises a forward error correction (FEC)encoder 404 which accepts the first signal symbols 402 and addsredundant information that are used to reduce transmission errors. Thecoded symbols 405 are modulated by modulator 406 according to a firstcarrier 408 to produce an upper layer modulated signal 410. Secondsymbols 420 are likewise provided to an optional second FEC encoder 422to produce coded second symbols 424. The coded second symbols 424 areprovided to a second modulator 414, which modulates the coded secondsymbols 424 according to a second carrier 416 to produce a lower layermodulated signal 418. The upper layer modulated signal 410 and the lowerlayer modulated signal 418 are therefore uncorrelated. Thus, the upperlayer signal 410 and the lower layer signal 418 can be transmitted toseparate transponders on one or more satellites 108 via separate uplinksignals 116. Thus, the lower layer signal 418 can be implemented from aseparate satellite 108 that receives a separate uplink signal 116.However, in the downlink signal 118 the upper layer signal 410, must bea sufficiently greater amplitude signal than the lower layer signal 418,to maintain the signal constellations shown in FIG. 6 and FIG. 7.

It should be noted that it may be more efficient to retrofit an existingsystem by using a transponder on a separate satellite 108 to transmitthe lower layer downlink signal over the existing legacy downlink signalrather than replacing the legacy satellite with one that will transmitboth downlink signal layers. Emphasis can be given to accommodating thedownlink legacy signal in implementing a layered downlink broadcast.

2.3 Integrated Receiver/Decoder

FIG. 5 is a block diagram of an integrated receiver/decoder (IRD) 500(also hereinafter alternatively referred to as receiver 500). Thereceiver 500 comprises a tuner/demodulator 504 communicatively coupledto an ODU 112 having one or more low noise blocks (LNBs) 502. The LNB502 converts the 12.2- to 12.7 GHz downlink 118 signal from thesatellites 108 to, e.g., a 950–1450 MHz signal required by the IRD's 500tuner/demodulator 504. Typically, the LNB 502 may provide either a dualor a single output. The single-output LNB 502 has only one RF connector,while the dual output LNB 502 has two RF output connectors and can beused to feed a second tuner 504, a second receiver 500, or some otherform of distribution system.

The tuner/demodulator 504 isolates a single, digitally modulated 24 MHztransponder signal, and converts the modulated data to a digital datastream. Further details regarding the demodulation of the receivedsignal follow.

The digital data stream is then supplied to a forward error correction(FEC) decoder 506. This allows the IRD 500 to reassemble the datatransmitted by the uplink center 104 (which applied the forward errorcorrection to the desired signal before transmission to the subscriberreceiving station 110) verifying that the correct data signal wasreceived, and correcting errors, if any. The error-corrected data may befed from the FEC decoder module 506 to the transport module 508 via an8-bit parallel interface.

The transport module 508 performs many of the data processing functionsperformed by the IRD 500. The transport module 508 processes datareceived from the FEC decoder module 506 and provides the processed datato the video MPEG decoder 514 and the audio MPEG decoder 517. As neededthe transport module employs system RAM 528 to process the data. In oneembodiment of the present invention, the transport module 508, videoMPEG decoder 514 and audio MPEG decoder 517 are all implemented onintegrated circuits. This design promotes both space and powerefficiency, and increases the security of the functions performed withinthe transport module 508. The transport module 508 also provides apassage for communications between the microcontroller 510 and the videoand audio MPEG decoders 514, 517. As set forth more fully hereinafter,the transport module also works with the conditional access module (CAM)512 to determine whether the subscriber receiving station 110 ispermitted to access certain program material. Data from the transportmodule 508 can also be supplied to external communication module 526.

The CAM 512 functions in association with other elements to decode anencrypted signal from the transport module 508. The CAM 512 may also beused for tracking and billing these services. In one embodiment of thepresent invention, the CAM 512 is a removable smart card, havingcontacts cooperatively interacting with contacts in the IRD 500 to passinformation. In order to implement the processing performed in the CAM512, the IRD 500, and specifically the transport module 508 provides aclock signal to the CAM 512.

Video data is processed by the MPEG video decoder 514. Using the videorandom access memory (RAM) 536, the MPEG video decoder 514 decodes thecompressed video data and sends it to an encoder or video processor 516,which converts the digital video information received from the videoMPEG module 514 into an output signal usable by a display or otheroutput device. By way of example, processor 516 may comprise a NationalTV Standards Committee (NTSC) or Advanced Television Systems Committee(ATSC) encoder. In one embodiment of the invention both S-Video andordinary video (NTSC or ATSC) signals are provided. Other outputs mayalso be utilized, and are advantageous if high definition programming isprocessed.

Audio data is likewise decoded by the MPEG audio decoder 517. Thedecoded audio data may then be sent to a digital to analog (D/A)converter 518. In one embodiment of the present invention, the D/Aconverter 518 is a dual D/A converter, one for the right and leftchannels. If desired, additional channels can be added for use insurround sound processing or secondary audio programs (SAPs). In oneembodiment of the invention, the dual D/A converter 518 itself separatesthe left and right channel information, as well as any additionalchannel information. Other audio formats may similarly be supported. Forexample, other audio formats such as multi-channel DOLBY DIGITAL AC-3may be supported.

A description of the processes performed in the encoding and decoding ofvideo streams, particularly with respect to MPEG and JPEGencoding/decoding, can be found in Chapter 8 of “Digital TelevisionFundamentals,” by Michael Robin and Michel Poulin, McGraw-Hill, 1998,which is hereby incorporated by reference herein.

The microcontroller 510 receives and processes command signals from theremote control 524, an IRD 500 keyboard interface, and/or another inputdevice. The microcontroller 510 receives commands for performing itsoperations from a processor programming memory, which permanently storessuch instructions for performing such commands. The processorprogramming memory may comprise a read only memory (ROM) 538, anelectrically erasable programmable read only memory (EPROM) 522 or,similar memory device. The microcontroller 510 also controls the otherdigital devices of the IRD 500 via address and data lines (denoted “A”and “D” respectively, in FIG. 5).

The modem 540 connects to the customer's phone line via the PSTN port120. It calls, e.g. the program provider, and transmits the customer'spurchase information for billing purposes, and/or other information. Themodem 540 is controlled by the microprocessor 510. The modem 540 canoutput data to other I/O port types including standard parallel andserial computer I/O ports.

The present invention also comprises a local storage unit such as thevideo storage device 532 for storing video and/or audio data obtainedfrom the transport module 508. Video storage device 532 can be a harddisk drive, a read/writable compact disc of DVD, a solid state RAM, orany other suitable storage medium. In one embodiment of the presentinvention, the video storage device 532 is a hard disk drive withspecialized parallel read/write capability so that data may be read fromthe video storage device 532 and written to the device 532 at the sametime. To accomplish this feat, additional buffer memory accessible bythe video storage 532 or its controller may be used. Optionally, a videostorage processor 530 can be used to manage the storage and retrieval ofthe video data from the video storage device 532. The video storageprocessor 530 may also comprise memory for buffering data passing intoand out of the video storage device 532. Alternatively or in combinationwith the foregoing, a plurality of video storage devices 532 can beused. Also alternatively or in combination with the foregoing, themicrocontroller 510 can also perform the operations required to storeand or retrieve video and other data in the video storage device 532.

The video processing module 516 input can be directly supplied as avideo output to a viewing device such as a video or computer monitor. Inaddition, the video and/or audio outputs can be supplied to an RFmodulator 534 to produce an RF output and/or 8 vestigal side band (VSB)suitable as an input signal to a conventional television tuner. Thisallows the receiver 500 to operate with televisions without a videooutput.

Each of the satellites 108 comprises a transponder, which acceptsprogram information from the uplink center 104, and relays thisinformation to the subscriber receiving station 110. Known multiplexingtechniques are used so that multiple channels can be provided to theuser. These multiplexing techniques include, by way of example, variousstatistical or other time domain multiplexing techniques andpolarization multiplexing. In one embodiment of the invention, a singletransponder operating at a single frequency band carries a plurality ofchannels identified by respective service channel identification (SCID).

Preferably, the IRD 500 also receives and stores a program guide in amemory available to the microcontroller 510. Typically, the programguide is received in one or more data packets in the data stream fromthe satellite 108. The program guide can be accessed and searched by theexecution of suitable operation steps implemented by the microcontroller510 and stored in the processor ROM 538. The program guide may includedata to map viewer channel numbers to satellite transponders and servicechannel identifications (SCIDs), and also provide TV program listinginformation to the subscriber 122 identifying program events.

The functionality implemented in the IRD 500 depicted in FIG. 5 can beimplemented by one or more hardware modules, one or more softwaremodules defining instructions performed by a processor, or a combinationof both.

The present invention provides for the modulation of signals atdifferent power levels and advantageously for the signals to benon-coherent from each layer. In addition, independent modulation andcoding of the signals may be performed. Backwards compatibility withlegacy receivers, such as a quadrature phase shift keying (QPSK)receiver is enabled and new services are provided to new receivers. Atypical new receiver of the present invention uses two demodulators andone remodulator as will be described in detail hereafter.

In a typical backwards-compatible embodiment of the present invention,the legacy QPSK signal is boosted in power to a higher transmission (andreception) level. This creates a power “room” in which a new lower layersignal may operate. The legacy receiver will not be able to distinguishthe new lower layer signal, from additive white Gaussian noise, and thusoperates in the usual manner. The optimum selection of the layer powerlevels is based on accommodating the legacy equipment, as well as thedesired new throughput and services.

The new lower layer signal is provided with a sufficient carrier tothermal noise ratio to function properly. The new lower layer signal andthe boosted legacy signal are non-coherent with respect to each other.Therefore, the new lower layer signal can be implemented from adifferent TWTA and even from a different satellite. The new lower layersignal format is also independent of the legacy format, e.g., it may beQPSK or 8PSK, using the conventional concatenated FEC code or using anew Turbo code. The lower layer signal may even be an analog signal.

The combined layered signal is demodulated and decoded by firstdemodulating the upper layer to remove the upper carrier. The stabilizedlayered signal may then have the upper layer FEC decoded and the outputupper layer symbols communicated to the upper layer transport. The upperlayer symbols are also employed in a remodulator, to generate anidealized upper layer signal. The idealized upper layer signal is thensubtracted from the stable layered signal to reveal the lower layersignal. The lower layer signal is then demodulated and FEC decoded andcommunicated to the lower layer transport.

Signals, systems and methods using the present invention may be used tosupplement a pre-existing transmission compatible with legacy receivinghardware in a backwards-compatible application or as part of apreplanned layered modulation architecture providing one or moreadditional layers at a present or at a later date.

2.4 Layered Signals

FIGS. 6A–6C illustrate the basic relationship of signal layers in areceived layered modulation transmission. FIG. 6A illustrates an upperlayer signal constellation 600 of a transmission signal showing thesignal points or symbols 602. FIG. 6B illustrates the lower layer signalconstellation of symbols 604 over the upper layer signal constellation600 where the layers are coherent (or synchronized). FIG. 6C illustratesa lower layer signal 606 of a second transmission layer over the upperlayer constellation where the layers are non-coherent. The lower layer606 rotates about the upper layer constellation 602 due to the relativemodulating frequencies of the two layers in a non-coherent transmission.Both the upper and lower layers rotate about the origin due to the firstlayer modulation frequency as described by path 608.

FIGS. 7A–7C are diagrams illustrating a non-coherent relationshipbetween a lower transmission layer over the upper transmission layerafter upper layer demodulation. FIG. 7A shows the constellation 700before the first carrier recovery loop (CRL) of the upper layer and theconstellation rings 702 rotate around the large radius circle indicatedby the dashed line. FIG. 7B shows the constellation 704 after CRL of theupper layer where the rotation of the constellation rings 702 isstopped. The constellation rings 702 are the signal points of the lowerlayer around the nodes 602 of the upper layer. FIG. 7C depicts a phasedistribution of the received signal with respect to nodes 602.

Relative modulating frequencies of the non-coherent upper and lowerlayer signals cause the lower layer constellation to rotate around thenodes 602 of the upper layer constellation to form rings 702. After thelower layer CRL this rotation is eliminated and the nodes of the lowerlayer are revealed (as shown in FIG. 6B). The radius of the lower layerconstellation rings 702 is indicative of the lower layer power level.The thickness of the rings 702 is indicative of the carrier to noiseratio (CNR) of the lower layer. As the two layers are non-coherent, thelower layer may be used to transmit distinct digital or analog signals.

FIG. 8A is a diagram showing a system for transmitting and receivinglayered modulation signals. Separate transmitters 107A, 107B (thatinclude TWTAs to amplify the signals), as may be located on any suitableplatform, such as satellites 108A, 108B, are used to non-coherentlytransmit different layers of a signal of the present invention. Uplinksignals 116 are typically transmitted to each satellite 108A, 108B fromone or more uplink centers 104 with one or more transmitters 105 via anantenna 106.

FIG. 8B is a diagram illustrating an exemplary satellite transponder 107for receiving and transmitting layered modulation signals on a satellite108. The uplink signal 116 is received by the satellite 108 and passedthrough a input multiplexer (IMUX) 814. Following this the signal isamplified with a travelling wave tube amplifier (TWTA) 816 and thenthrough an output muliplexer (OMUX) 818 before the downlink signal 118is transmitted to the receivers 802, 500.

The layered signals 808A, 808B (e.g. multiple downlink signals 118) arereceived at receiver antennas 812A, 812B, such as satellite dishes, eachwith a low noise block (LNB) 810A, 8101B where they are then coupled tointegrated receiver/decoders (IRDs) 500, 802. For example, firstsatellite 108A and transmitter 107A can transmit an upper layer legacysignal 808A and second satellite 108B and transmitter 107B can transmita lower layer signal 808B. Although both signals 808A, 808B arrive ateach antenna 812A, 812B and LNB 810A, 810B, only the layer modulationIRD 802 is capable of decoding both signals 808A, 808B. The legacyreceiver 500 is only capable of decoding the upper layer legacy signal808A; the lower layer signal 808B appears only as noise to the legacyreceiver 500.

Because the signal layers may be transmitted non-coherently, separatetransmission layers may be added at any time using different satellites108A, 108B or other suitable platforms, such as ground based or highaltitude platforms. Thus, any composite signal, including new additionalsignal layers will be backwards compatible with legacy receivers 500,which will disregard the new signal layers. To ensure that the signalsdo not interfere, the combined signal and noise level for the lowerlayer must be at or below the allowed noise floor for the upper layer atthe particular receiver antenna 812A, 812B.

Layered modulation applications include backwards compatible andnon-backwards compatible applications. “Backwards compatible” in thissense, describes systems in which legacy receivers 500 are not renderedobsolete by the additional signal layer(s). Instead, even if the legacyreceivers 500 are incapable of decoding the additional signal layer(s),they are capable of receiving the layered modulated signal and decodingthe original signal layer. In these applications, the pre-existingsystem architecture is accommodated by the architecture of theadditional signal layers. “Non-backwards compatible” describes a systemarchitecture which makes use of layered modulation, but the modulationscheme employed is such that pre-existing equipment is incapable ofreceiving and decoding the information on additional signal layer(s).

The pre-existing legacy IRDs 500 decode and make use of data only fromthe layer (or layers) they were designed to receive, unaffected by theadditional layers. However, as will be described hereafter, the legacysignals may be modified to optimally implement the new layers. Thepresent invention may be applied to existing direct satellite serviceswhich are broadcast to individual users in order to enable additionalfeatures and services with new receivers without adversely affectinglegacy receivers and without requiring additional signal frequency.

2.5 Demodulator and Decoder

FIG. 9 is a block diagram depicting one embodiment of an enhanced IRD802 capable of receiving layered modulation signals. The enhanced IRD802 includes a feedback path 902 in which the FEC decoded symbols arefed back to a enhanced modified tuner/demodulator 904 and transportmodule 908 for decoding both signal layers as detailed hereafter.

FIG. 10A is a block diagram of one embodiment of the enhancedtuner/modulator 904 and FEC encoder 506. FIG. 10A depicts receptionwhere layer subtraction is performed on a signal where the upper layercarrier has already been demodulated. The upper layer of the receivedcombined signal 1016 from the LNB 502, which may contain legacymodulation format, is provided to and processed by an upper layerdemodulator 1004 to produce the stable demodulated signal 1020. Thedemodulated signal 1010 is communicatively coupled to a FEC decoder 1002which decodes the upper layer to produce the upper layer symbols whichare output to an upper layer transport module 908. The upper layersymbols are also used to generate an idealized upper layer signal. Theupper layer symbols may be produced from the decoder 402 after Viterbidecode (BER<10⁻³ or so) or after Reed-Solomon (RS) decode (BER<10⁻⁹ orso), in typical decoding operations known to those skilled in the art.The upper layer symbols are provided via feedback path 902 from theupper layer decoder 402 to a remodulator 406 which effectively producesan idealized upper layer signal. The idealized upper level signal issubtracted from the demodulated upper layer signal 1020.

In order for the subtraction to yield a clean small lower layer signal,the upper layer signal must be precisely reproduced. The modulatedsignal may have been distorted, for example, by traveling wave tubeamplifier (TWTA) non-linearity or other non-linear or linear distortionsin the transmission channel. The distortion effects are estimated fromthe received signal after the fact or from TWTA characteristics whichmay be downloaded into the IRD in AM-AM and/or AM-PM maps 1014, used toeliminate the distortion (e.g., using the non-linear distortion mapmodule 1018) (see detailed description below).

A subtractor 1012 then subtracts the idealized upper layer signal fromthe stable demodulated signal 1020. This leaves the lower-power secondlayer signal. The subtractor 1012 may include a buffer or delay functionto retain the stable demodulated signal 1020 while the idealized upperlayer signal is being constructed. The second layer signal isdemodulated by the lower level demodulator 1010 and FEC decoded bydecoder 1008 according to its signal format to produce the lower layersymbols, which are provided to the transport module 908.

FIG. 10B depicts another embodiment wherein layer subtraction isperformed on the received layered signal (prior to upper layerdemodulation). In this case, the upper layer demodulator 1004 producesthe upper carrier signal 1022 (as well as the stable demodulated signaloutput 1020). An upper carrier signal 1022 is provided to theremodulator 1006. The remodulator 1006 provides the remodulated signalto the non-linear distortion mapper 1018 which effectively produces anidealized upper layer signal. Unlike the embodiment shown in FIG. 10A,in this embodiment, the idealized upper layer signal includes the upperlayer carrier for subtraction from the received combined signal 808A,808B.

Other equivalent methods of layer subtraction will occur to thoseskilled in the art and the present invention should not be limited tothe examples provided here. Furthermore, those skilled in the art willunderstand that the present invention is not limited to two layers;additional layers may be included. Idealized upper layers are producedthrough remodulation from their respective layer symbols and subtracted.Subtraction may be performed on either the received combined signal or ademodulated signal. Finally, it is not necessary for all signal layersto be digital transmissions; the lowest layer may be an analogtransmission.

The following analysis describes the exemplary two layer demodulationand decoding. It will be apparent to those skilled in the art thatadditional layers may be demodulated and decoded in a similar manner.The incoming combined signal is represented as:

$\begin{matrix}{{s_{UL}(t)} = {{f_{U}\;\left( {M_{U}\exp\mspace{11mu}\left( {{{j\omega}_{U}t} + \theta_{U}} \right){\sum\limits_{m = {- \infty}}^{\infty}\;{S_{Um}p\mspace{11mu}\left( {t - {mT}} \right)}}} \right)} +}} \\{{f_{L}\left( {M_{L}\exp\mspace{11mu}\left( {{{j\omega}_{L}t} + \theta_{L}} \right){\sum\limits_{m = {- \infty}}^{\infty}\;{S_{Lm}p\mspace{11mu}\left( {t - {mT} + {\Delta\; T_{m}}} \right)}}} \right)} + {n\mspace{11mu}(t)}}\end{matrix}$where, M_(U) is the magnitude of the upper layer QPSK signal and M_(L)is the magnitude of the lower layer QPSK signal and M_(L)<<M_(U). Thesignal frequencies and phase for the upper and lower layer signals arerespectively ω_(U),θ_(U) and ω_(L),θ_(L). The symbol timing misalignmentbetween the upper and lower layers is ΔT_(m). p(t−mT) represents thetime shifted version of the pulse shaping filter p(t) 414 employed insignal modulation. QPSK symbols S_(Um) and S_(Lm) are elements of

$\left\{ {{\exp\;\left( {j\frac{n\;\pi}{2}} \right)},{n = 0},1,2,3} \right\} \cdot {f_{U}( \cdot )}$and ƒ_(L)(·) denote the distortion function of the TWTAs for therespective signals.

Ignoring ƒ_(U)(·) and ƒ_(L)(·) and noise n(t), the following representsthe combined signal after removing the upper carrier:

$\begin{matrix}{{{s^{\prime}}_{UL}(t)} = {{M_{U}{\sum\limits_{m = {- \infty}}^{\infty}\;{S_{Um}p\mspace{11mu}\left( {t - {mT}} \right)}}} + {M_{L}\exp}}} \\{\left\{ {{j\mspace{11mu}\left( {\omega_{L} - \omega_{U}} \right)\mspace{11mu} t} + \theta_{L} - \theta_{U}} \right\}{\sum\limits_{m = {- \infty}}^{\infty}\;{S_{Lm}p\mspace{11mu}\left( {t - {mT} + {\Delta\; T_{m}}} \right)}}}\end{matrix}$Because of the magnitude difference between M_(U) and M_(L), the upperlayer demodulator 1004 and decoder 1002 disregard the M_(L) component ofthe s′_(UL)(t).

After subtracting the upper layer from s_(UL)(t) in the subtractor 1012,the following remains:

${s_{L}(t)} = {M_{L}\exp\mspace{11mu}\left\{ {{j\mspace{11mu}\left( {\omega_{L} - \omega_{U}} \right)\mspace{11mu} t} + \theta_{L} - \theta_{U}} \right\}{\sum\limits_{m = {- \infty}}^{\infty}\;{S_{Lm}p\mspace{11mu}\left( {t - {mT} + {\Delta\; T_{m}}} \right)}}}$Any distortion effects, such as TWTA nonlinearity effects are estimatedfor signal subtraction. In a typical embodiment of the presentinvention, the upper and lower layer frequencies are substantiallyequal. Significant improvements in system efficiency can be obtained byusing a frequency offset between layers.

Using the present invention, two-layered backward compatible modulationwith QPSK doubles a current 6/7 rate capacity by adding a TWTAapproximately 6.2 dB above an existing TWTA power. New QPSK signals maybe transmitted from a separate transmitter, from a different satellitefor example. In addition, there is no need for linear travelling wavetube amplifiers (TWTAs) as with 16QAM. Also, no phase error penalty isimposed on higher order modulations such as 8PSK and 16QAM.

3.0 Power Levels of Modulation Layers

In a layered modulation system, the relationship between the individualmodulation layers can be structured to facilitate backward compatibleapplications. Alternately, a new layer structure can be designed tooptimize the combined efficiency and/or performance of the layeredmodulation system.

3.1 Backward Compatible Applications

The present invention may be used in Backward Compatible Applications.In such applications, a lower layer signal may take advantage ofadvanced forward error correction (FEC) coding techniques to lower theoverall transmission power required by the system.

FIG. 11A depicts the relative power levels 1100 of example embodimentsof the present invention. FIG. 11A is not a scale drawing. Thisembodiment doubles the pre-existing rate 6/7 capacity by using a TWTA6.2 dB above a pre-existing TWTA equivalent isotropic radiated power(ERP) and second TWTA 2 dB below the pre-existing TWTA power. Thisembodiment uses upper and lower QPSK layers which are non-coherent. Acode rate of 6/7 is also used for both layers. In this embodiment, thesignal of the legacy QPSK signal 1102 is used to generate the upperlayer 1104 and a new QPSK layer is the lower layer 1110. The CNR of thelegacy QPSK signal 1102 is approximately 7 dB. In the present invention,the legacy QPSK signal 1102 is boosted in power by approximately 6.2 dBbringing the new power level to approximately 13.2 dB as the upper layer1104. The noise floor 1106 of the upper layer is approximately 6.2 dB.The new lower QPSK layer 1110 has a CNR of approximately 5 dB. The totalsignal and noise of the lower layer is kept at or below the tolerablenoise floor 1106 of the upper layer. The power boosted upper layer 1104of the present invention is also very robust, making it resistant torain fade. It should be noted that the invention may be extended tomultiple layers with mixed modulations, coding and code rates.

In an alternate embodiment of this backwards compatible application, acode rate of ⅔ may be used for both the upper and lower layers 1104,1110. In this case, the CNR of the legacy QPSK signal 1102 (with a coderate of ⅔) is approximately 5.8 dB. The legacy signal 1102 is boosted byapproximately 5.3 dB to approximately 11.1 dB (4.1 dB above the legacyQPSK signal 1102 with a code rate of ⅔) to form the upper QPSK layer1104. The new lower QPSK layer 1110 has a CNR of approximately 3.8 dB.The total signal and noise of the lower layer 1110 is kept at or belowapproximately 5.3 dB, the tolerable noise floor 1106 of the upper QPSKlayer. In this case, overall capacity is improved by 1.55 and theeffective rate for legacy IRDs will be 7/9 of that before implementingthe layered modulation.

In a further embodiment of a backwards compatible application of thepresent invention the code rates between the upper and lower layers1104, 1110 may be mixed. For example, the legacy QPSK signal 502 may beboosted by approximately 5.3 dB to approximately 12.3 dB with the coderate unchanged at 6/7 to create the upper QPSK layer 1104. The new lowerQPSK layer 1110 may use a code rate of ⅔ with a CNR of approximately 3.8dB. In this case, the total capacity relative to the legacy signal 1102is approximately 1.78. In addition, the legacy IRDs will suffernosignificant rate decrease.

3.2 Non-Backward Compatible Applications

As previously discussed the present invention may also be used in“non-backward compatible” applications. In such applications, both upperand lower layer signals may take advantage of advanced forward errorcorrection (FEC) coding techniques to lower the overall transmissionpower required by the system. In a first example embodiment, two QPSKlayers 1104, 1110 are used each at a code rate of ⅔. The upper QPSKlayer 504 has a CNR of approximately 4.1 dB above its noise floor 1106and the lower QPSK layer 1110 also has a CNR of approximately 4.1 dB.The total code and noise level of the lower QPSK layer 1110 isapproximately 5.5 dB. The total CNR for the upper QPSK signal 1104 isapproximately 9.4 dB, merely 2.4 dB above the legacy QPSK signal rate6/7. The capacity is approximately 1.74 compared to the legacy rate 6/7.

FIG. 11B depicts the relative power levels of an alternate embodimentwherein both the upper and lower layers 1104, 1110 are below the legacysignal level 1102. The two QPSK layers 1104, 1110 use a code rate of ½.In this example, the upper QPSK layer 1104 is approximately 2.0 dB aboveits noise floor 1106 of approximately 4.1 dB. The lower QPSK layer has aCNR of approximately 2.0 dB and a total code and noise level at or below4.1 dB. The capacity of this embodiment is approximately 1.31 comparedto the legacy rate 6/7.

4. Hardware Environment

FIG. 12 illustrates an exemplary computer system 1200 that could be usedto implement selected modules and/or functions of the present invention.The computer 1202 comprises a processor 1204 and a memory 1206, such asrandom access memory (RAM). The computer 1202 is operatively coupled toa display 1222, which presents images such as windows to the user on agraphical user interface 1218B. The computer 1202 may be coupled toother devices, such as a keyboard 1214, a mouse device 1216, a printer,etc. Of course, those skilled in the art will recognize that anycombination of the above components, or any number of differentcomponents, peripherals, and other devices, may be used with thecomputer 1202.

Generally, the computer 1202 operates under control of an operatingsystem 1208 stored in the memory 1206, and interfaces with the user toaccept inputs and commands and to present results through a graphicaluser interface (GUI) module 1218A. Although the GUI module 1218A isdepicted as a separate module, the instructions performing the GUIfunctions can be resident or distributed in the operating system 1208,the computer program 1210, or implemented with special purpose memoryand processors. The computer 1202 also implements a compiler 1212 whichallows an application program 1210 written in a programming languagesuch as COBOL, C++, FORTRAN, or other language to be translated intoprocessor 1204 readable code. After completion, the application 1210accesses and manipulates data stored in the memory 1206 of the computer1202 using the relationships and logic that was generated using thecompiler 1212. The computer 1202 also optionally comprises an externalcommunication device such as a modem, satellite link, Ethernet card, orother device for communicating with other computers.

In one embodiment, instructions implementing the operating system 1208,the computer program 1210, and the compiler 1212 are tangibly embodiedin a computer-readable medium, e.g., data storage device 1220, whichcould include one or more fixed or removable data storage devices, suchas a zip drive, floppy disc drive 1224, hard drive, CD-ROM drive, tapedrive, etc. Further, the operating system 1208 and the computer program1210 are comprised of instructions which, when read and executed by thecomputer 1202, causes the computer 1202 to perform the steps necessaryto implement and/or use the present invention. Computer program 1210and/or operating instructions may also be tangibly embodied in memory1206 and/or data communications devices 1230, thereby making a computerprogram product or article of manufacture according to the invention. Assuch, the terms “article of manufacture,” “program storage device” and“computer program product” as used herein are intended to encompass acomputer program accessible from any computer readable device or media.

Those skilled in the art will recognize many modifications may be madeto this configuration without departing from the scope of the presentinvention. For example, those skilled in the art will recognize that anycombination of the above components, or any number of differentcomponents, peripherals, and other devices, may be used with the presentinvention.

5. Estimating the Operating Point

Referring again to FIGS. 10A and 10B, non-linear distortion maps 1018that depict the non-linearity of the TWTA may be used by a non-lineardistortion map module during the layered modulation signalreconstruction process. However, it may be difficult to accuratelydetermine the non-linearity and operating point of the TWTA (e.g., fromreceived data in satellite communication) to produce a high-fidelityreconstructed signal, particularly for layered modulation applications.In this regard, as described above, in an exemplary receiver 802, a TWTAAM-AM and AM-PM map are applied (e.g., using an estimated operatingpoint) to a re-encoded and re-modulated signal to more accuratelyreconstruct the upper layer signal.

While FIG. 10 illustrates the use of the non-linear distortion maps,knowledge of the non-linear distortion maps and operating point must bedetermined. FIG. 13 is a flow chart illustrating the determination ofthe operating point in accordance with one or more embodiments of theinvention. At step 1302, TWTA non-linearity (i.e., the AM-AM and AM-PMcurves) is measured (e.g. on-line). TWTA non-linearity may be measuredin a variety of manners as described in further detail below.

Regardless of the technique used to measure TWTA non-linearity, theroot-mean-squared (RMS) value of the input signal at the time of thenon-linearity measurement (used to measure the curves) is computed atstep 1304. The input signal refers to the reconstructed clean signalbefore the imposition of TWTA nonlinearity The RMS value identifies theinput operating point on the measured nonlinearity curves.

The output operating point is then obtained at step 1306 (e.g., as abyproduct of the non-linearity measurement data). The output operatingpoint may be obtained using a variety of methods. For example, theoutput operating point may be calculated from the RMS value of theoutput (received) values used to determine the TWTA non-linearity curve(e.g., when matching the curve as described below) less the estimatednoise power value. The output operating point may also be obtained fromthe corresponding point on the measured TWTA non-linearity curves. Withthe input and output operating points obtained, the upper layer signal(of a layered modulation) may be more accurately reconstructed as partof the layered modulation scheme.

It should be noted that the measurement of non-linearity (i.e., step1302) may be conducted in a variety of manners as part of the layeredmodulation scheme. Nonetheless, regardless of the technique used tomeasure non-linearity, the operating point is estimated along with themeasurement for the non-linearity curves. The TWTA non-linearity may bemeasured at the local IRDs 500, in which case the operating point may beautomatically calculated from the nonlinearity measurements. The TWTAnon-linearity may also be made at a broadcast/uplink center 104 with theoperating point similarly obtained, in which case information on TWTAnon-linearity and operating point can be downloaded to individual IRDs500, such as through the downlink signal 118, to support the layeredmodulation signal receiving process.

6. Measuring Non-Linearity

As described above, the measurement of non-linearity (i.e., step 1302)may be conducted in a variety of manners as part of the layeredmodulation scheme. A first mechanism for TWTA non-linearity measurementis fully described in U.S. patent application Ser. No. 10/165,710,entitled “SATELLITE TWTA ONLINE NON-LINEARITY MEASUREMENT”, filed onJun. 7, 2002 by Ernest C. Chen. A second measurement mechanism is fullydescribed in U.S. Provisional Patent Application Ser. No. 60/510,368,entitled “IMPROVING TWTA AM-AM AND AM-PM MEASUREMENT”, filed on Oct. 10,2003, by Ernest C. Chen. The second mechanism represents an improvementover the first mechanism. Non-linearity may be measured in each localIRD 500 (e.g., using a coherent averaging technique that maximizessignal processing gains).

TWTA non-linearity may be measured locally within individual IRDs. Thismay, eliminate the need to transmit the non-linearity curves from thebroadcast/uplink center 104. TWTA non-linearity can also be measured atthe broadcast/uplink center 104 using a similar estimation procedure asthat described above but possibly with a larger receive antenna forincreased CNR as desired. The IRD 802 which receives the downlink signal118 (e.g., from the LNB 502) may also include a signal processor whichextracts the symbol stream and carrier frequency from the incomingsignal and generates an ideal signal, i.e. a signal without the effectsof the TWTA and noise. The ideal signal is then used in a comparisonprocessor to produce TWTA characteristic maps (which provide themeasurements for TWTA non-linearity). As described herein, the signalprocessor and comparison processor may be incorporated in IRD 802 withinthe tuner/demodulator 904, FEC 506. The details concerning thegeneration of the characteristic maps will be described below in thediscussion of FIGS. 14A–14C.

Typically, the TWTA characteristic maps comprise measurements of theoutput amplitude modulation versus the input amplitude modulation (theAM-AM map) and the output phase modulation versus the input amplitudemodulation (the AM-PM map). The received signal represents the TWTAamplifier output (plus noise) and the generated ideal signal representsthe amplifier input. In addition to diagnosing and monitoring theamplifier, these characteristic maps may then be used to facilitateand/or improve reception of lower layer signals of a system using alayered modulation transmission scheme.

FIGS. 14A and 14B are block diagrams of the basic system 1400 formeasuring the characteristic maps. All of the described functions may becarried out within a receiver 802 used in a direct broadcast satellitesystem having a basic architecture as described above. The appropriatesignal section is captured and demodulated by demodulator 1402 whichaligns symbol timing and removes any residual carrier frequency andphase in the signal. The demodulated signal is used in a signalgenerator 1404 to generate an ideal signal, i.e. one representing thepre-transmitted signal. In the case of a digital signal, the signal willbe further decoded to obtain the signal symbols which will be used togenerate the ideal signal. The difference between the ideal signal andthe received signal is used by processors 1406, 1410, 1408, 1412 toestimate a transmission non-linearity characteristic. Only a smallsection of the received signal, on the order of a few thousand symbols,may be needed to obtain an estimate.

FIG. 14A depicts an embodiment where the non-linearity characteristic isestimated from a difference between the generated ideal signal(noise-free and without TWTA non-linearity) and the received signalafter demodulation. Because the ideal signal is generated from only thesymbols and symbol timing, obtaining the estimate from the receivedsignal after demodulation simplifies the processing.

FIG. 14B depicts an embodiment where the performance characteristic isestimated from a difference between the ideal signal and the receivedsignal before demodulation. In this case, the ideal signal must also begenerated with the carrier frequency of the received signal. This may bedone by adding the demodulated symbol timing and carrier frequency andphase to the ideal signal.

If necessary, forward error correction (FEC) may be applied to thedemodulated signal as part of decoding to ensure that all recoveredsymbols are error-free.

In either embodiment (FIG. 14A or 14B) the ideal signal and the receivedsignal are next used in processors 1406, 1408 to pair and sort datapoints of the two signals in a two-dimensional scattergram (ascattergram for purposes herein is the collection of paired points withinput and output values represented along X and Y axes, respectively).These processors 1406, 1408 characterize a relationship between an inputsignal and an output signal of the amplifier plus noise. In this case,the input signal is represented by the generated ideal signal 1420(re-modulated or otherwise) and the output signal is represented by thereceived signal. The X-axis of an AM-AM scattergram plots the magnitudesof the ideal signal samples with perfect TWTA linearity, and the Y-axisconsists of the magnitudes of the received signal samples including theTWTA non-linearity (and noise). An AM-PM scattergram is similarlyformed. The X-axis is the same as that for the AM-AM scattergram, andthe Y-axis consists of all phase differences between the correspondingsamples with and without TWTA non-linearity. Finally, the data points ofthe ideal signal and the corresponding data points of the receivedsignal are processed by a processor 1410, 1412 to form a line throughcurve fitting, such as with a polynomial. The curve fitting processor1410, 1412 may be separate or part of the processor 1406, 1408 whichpaired and sorted the data points. The result is an estimate of thedesired performance characteristic of the TWTA 1414, 1416.

FIG. 14C outlines the flow of a method of the present invention. Asignal is received at block 1422. The signal is demodulated at block1424. Then an ideal signal is generated from the demodulated signal atblock 1426. Finally, a performance characteristic (i.e., a TWTAnon-linearity curve) is estimated from a difference between the idealsignal and the received signal at block 1428.

7. Offsetting (Shifting) the TWTA Non-Linearity Measurements

Independent of the operating point estimation described above, themeasured AM-AM and AM-PM curves may be deliberately offset or shifted tosimplify the reconstruction of the upper layer signal during the signalreconstruction and cancellation process. Such an offset does not alterthe performance of layered modulation processing (or non-linearitycompensation performance). In fact, offsetting the operating point mayresult in a simple and consistent representation of TWTA non-linearityregardless of input saturation, input backoff, etc.

To offset the measurement curves, the input and output amplitude values(i.e., used during the non-linearity curve measurement) may be rescaledso that the operating point is at a desired reference point (e.g., 0dB), for both input and output (e.g., thereby providing referencedoperating point values). In the log domain, such resealing may beperformed by subtracting the measured (AM) input operating point value(in dB) from all input values (in dB). Likewise, the measured output(AM) operating point value (in dB) may be subtracted from values of alloutput points (in dB). Thus, by offsetting the measurement curves, thecurves may be more easily referenced. In silicon and other hardwareimplementations, however, it may be desirable to scale the input andoutput operating points or signals back (e.g., to −3 dB or −5 dB) toavoid signal saturation or fractional value representation overflow forincoming and outgoing signals. The shifting process can be donesimilarly to that described above.

With a shifted AM scale as desired, the output PM value may also berescaled by subtracting the measured (angular) phase value at the outputoperating point from the phase value of all output points.

The results of the above scaling is that the operating point willprovide reference values, such as (0 dB, 0 dB) for the AM-AM map, and (0dB, 0) for the AM-PM map. In this case the input signal must be scaledto 0 dB to match the operating point. To guard against signal saturationerrors (and to avoid the need for a look-up-table [LUT] extrapolation),bounding points may be placed beyond the measured signal interval toallow interpolation of the input data (or output testing data) in thetesting process that falls outside of the range of a TWTA measurementtable. The values for the bounding points may be obtained byextrapolating or replicating values from the endpoints of the TWTAmeasurement table

8. Signal Reconstruction with Complex Number Multiplications

Signal reconstruction with TWTA non-linearity, as described above, maybe efficiently achieved with complex number multiplications. FIG. 15Aillustrates an intuitive algorithm for obtaining the output in-phase andquadrature components (I_(o), Q_(o)) (that may be accomplished withseparate amplitude 1502 and phase 1504 corrections as indicated) fromthe input in-phase and quadrature components (I_(i), Q_(i)). In FIG.15A, the TWTA non-linearity effect is emulated with a multiplication oninput data. As illustrated, the input in-phase and quadrature componentsare processed through computationally-intensive rectangular-to-polartransformations 1506 (and the inverse 1508). The non-linearity is firstrepresented by two tables 1502 and 1504:

-   -   Table 1502=AM-AM:M_(i)        M_(o);    -   Table 1504=AM-PM:M_(i)        Δθ, such that        M_(i) exp(jθ_(i))        M_(o) exp(j(θ_(i)+Δθ))        Thus, the amplitude lookup table 1502 is used to produce an        output magnitude M_(o). The above equation may be equated with        the desired output expression:        M _(o) exp(j(θ_(i)+Δθ))=M _(i) exp(jθ _(i))M _(w) exp(jθ _(w))        (where M _(w) exp(jθ _(w)) is the multiplier for distortion)        where

$M_{w} = {{\frac{M_{o}}{M_{i}}{and}\mspace{14mu}\theta_{w}} = {{\Delta\theta}.}}$(Likewise, pre-distortion would be achieved by exp(−jθ_(w))/M_(w)).Accordingly, the phase lookup table 1504 is used to produce the changein output phase Δθ. The change in output phase Δθ is then added to theinput phase θ_(i) to produce the output phase θ_(o). The outputmagnitude M_(o) and output phase θ_(o) are then processed through apolar-to-rectangular transformation 1508 to produce the output in-phaseand quadrature components (I_(o), Q_(o)).

FIG. 15 B illustrates a computationally efficient algorithm forobtaining the input and output operating points (I₀, Q₀). In FIG. 15B,the LUTs (in (M, θ)) 1502 and 1504 (in FIG. 15A) are replaced with onecomplex multiplier LUT 1510 in (I,Q). The entries of the LUT iscalculated from:I _(w) +jQ _(w) =M _(w) exp(jΔθ _(w))The efficient algorithm begins with at 1512 with a computation of thesignal power. The signal power computation 1512 is followed by a tablelookup 1510 using the input signal power to index the proper complexmultiplier (I_(w), Q_(w)). The complex multiplier (I_(w), Q_(w)) is thenmultiplied with the incoming complex-valued data to effect the TWTAdistortion distortion:I _(o) +jQ _(o)=(I _(i) +jQ _(i))(I _(w) +jQ _(w))

Accordingly, the efficient scheme of FIG. 15B avoids computationallyintensive rectangular-to-polar and inverse transformations 1506 and 1508and requires simple power formation 1512 and a complex numbermultiplication through an LUT 1510. In addition, the efficient schememay include a complex matching factor in the complex multiplier table1510, the matching factor being the magnitude and phase differencebetween the upper and lower layer components of a layered modulationsignal if desired.

CONCLUSION

This concludes the description of the preferred embodiments of thepresent invention. The foregoing description of the preferred embodimentof the invention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching. For example, itis noted that the uplink configurations depicted and described in theforegoing disclosure can be implemented by one or more hardware modules,one or more software modules defining instructions performed by aprocessor, or a combination of both.

It is intended that the scope of the invention be limited not by thisdetailed description, but rather by the claims appended hereto. Theabove specification, examples and data provide a complete description ofthe manufacture and use of the apparatus and method of the invention.Since many embodiments of the invention can be made without departingfrom the scope of the invention, the invention resides in the claimshereinafter appended.

1. A method for determining an input operating point and an outputoperating point on a non-linear traveling wave tube amplifier (TWTA),comprising: measuring non-linearity of the TWTA; computing an inputroot-mean-square (RMS) value of an input signal used to measure thenon-linearity of the TWTA, wherein the input RMS value identifies aninput operating point of the measured non-linearity of the TWTA; andobtaining an output operating point.
 2. The method of claim 1, whereinthe measuring the non-linearity of the TWTA comprises measuring thenon-linearity at a local receiver.
 3. The method of claim 1, wherein themeasuring the non-linearity of the TWTA comprises measuring thenon-linearity at a broadcast center.
 4. The method of claim 3, furthercomprising downloading the measured non-linearity and the outputoperating point to an individual receiver.
 5. The method of claim 1,wherein obtaining the output operating point comprises calculating anoutput RMS value of output signals used in measuring the non-linearityof the TWTA.
 6. The method of claim 1, wherein obtaining the outputoperating point comprises obtaining a corresponding point on themeasured TWTA non-linearity based on the input RMS value.
 7. The methodof claim 1, further comprising reconstructing an upper layer signal of alayered modulation based on the output operating point.
 8. The method ofclaim 1, further comprising offsetting the measured non-linearity toprovide referenced operating point values.
 9. The method of claim 8,wherein the offsetting comprises scaling an input amplitude value andoutput amplitude value of the measured non-linearity of the TWTA toplace the input and output operating points at desired points.
 10. Themethod of claim 9, wherein the scaling comprises subtracting a measuredinput operating point value from all input values in a log domain. 11.The method of claim 9, wherein the scaling comprises subtracting ameasured output operating point value from all output values in a logdomain.
 12. The method of claim 9, wherein the scaling comprisessubtracting a measured phase value at the output operating point fromphase values of all output points used to measure the non-linearity ofthe TWTA.
 13. The method of claim 9, wherein the scaling furthercomprises: placing bounding points beyond end points used to measure thenon-linearity; and interpolating output testing data that falls outsideof the measured non-linearity based on the bounding points.
 14. Themethod of claim 8, further comprising mapping the input operating pointand output operating point to a particular level to avoid signalsaturation or fractional value representation overflow.
 15. An apparatusfor determining an input operating point and an output operating pointon a non-linear traveling wave tube amplifier (TWTA), comprising: meansfor measuring a non-linearity of the TWTA; means for computing an inputroot-mean-square (RMS) value of an input signal used to measure thenonlinearity of the TWTA, wherein the input RMS value identifies aninput operating point of the measured non-linearity of the TWTA; andmeans for obtaining an output operating point.
 16. The apparatus ofclaim 15, wherein the means for measuring the non-linearity of the TWTAcomprises means for measuring the non-linearity at a local receiver. 17.The apparatus of claim 15, wherein the means for measuring thenon-linearity of the TWTA comprises means for measuring thenon-linearity at a broadcast center.
 18. The apparatus of claim 17,further comprising means for downloading the measured non-linearity andthe output operating point to an individual receiver.
 19. The apparatusof claim 15, wherein the means for obtaining the output operating pointcomprises means for calculating an output RMS value of output signalsused in measuring the non-linearity of the TWTA.
 20. The apparatus ofclaim 15, wherein the means for obtaining the output operating pointcomprises means for obtaining a corresponding point on the measured TWTAnon-linearity based on the input RMS value.
 21. The apparatus of claim15, further comprising means for reconstructing an upper layer signal ofa layered modulation based on the output operating point.
 22. Theapparatus of claim 15, further comprising means for offsetting themeasured non-linearity to provide referenced operating point values. 23.The apparatus of claim 22, wherein the means for offsetting comprisesmeans for scaling an input amplitude value and output amplitude value ofthe measured non-linearity of the TWTA to place the input and outputoperating point at desired points.
 24. The apparatus of claim 23,wherein the means for scaling comprises means for subtracting a measuredinput operating point value from all input values in a log domain. 25.The apparatus of claim 23, wherein the means for scaling comprises meansfor subtracting a measured output operating point value from all outputvalues in a log domain.
 26. The apparatus of claim 23, wherein the meansfor scaling comprises means for subtracting a measured phase value atthe output operating point from phase values of all output points usedto measure the non-linearity of the TWTA.
 27. The apparatus of claim 23,wherein the means for scaling further comprises: means for placingbounding points beyond end points used to measure the non-linearity; andmeans for interpolating output testing data that falls outside of themeasured non-linearity based on the bounding points.
 28. The apparatusof claim 22, further comprising means for mapping the input operatingpoint and output operating point to a particular level to avoid signalsaturation or fractional value representation overflow.
 29. A system fordetermining an input operating point and an output operating point on anon-linear traveling wave tube amplifier (TWTA), comprising: (a) ameasuring module configured to: (1) measure non-linearity of the TWTA;and (2) obtaining an output operating point; and (b) a non-lineardistortion map module configured to compute an input root-mean-square(RMS) value of an input signal used to measure the non-linearity of theTWTA, wherein the RMS value identifies an input operating point of themeasured non-linearity of the TWTA.
 30. The system of claim 29, whereinthe measuring module is located at a local receiver.
 31. The system ofclaim 29, wherein the measuring module is located at a broadcast center.32. The system of claim 31, further comprising a receiver configured todownload the measured non-linearity and the output operating point. 33.The system of claim 29, wherein the measuring module is configured toobtain the output operating point by calculating an output RMS value ofoutput signals used in measuring the non-linearity of the TWTA.
 34. Thesystem of claim 29, wherein the measuring module is configured to obtainthe output operating point by obtaining a corresponding point on themeasured TWTA non-linearity based on the input RMS value.
 35. The systemof claim 29, further comprising a receiver configured to reconstruct anupper layer signal of a layered modulation based on the output operatingpoint.
 36. The system of claim 29, further comprising a receiverconfigured to offset the measured non-linearity to provide referencedoperating point values.
 37. The system of claim 36, wherein the receiveris configured to offset the measured non-linearity by scaling an inputamplitude value and output amplitude value of the measured non-linearityof the TWTA to place the input and output operating point at desiredpoints.
 38. The system of claim 37, wherein the receiver is configuredto scale by subtracting a measured input operating point value from allinput values in a log domain.
 39. The system of claim 37, wherein thereceiver is configured to scale by subtracting a measured outputoperating point value from all output values in a log domain.
 40. Thesystem of claim 37, wherein the receiver is configured to scale bysubtracting a measured phase value at the output operating point fromphase values of all output points used to measure the non-linearity ofthe TWTA.
 41. The system of claim 37, wherein the receiver is furtherconfigured to scale by: placing bounding points beyond end points usedto measure the non-linearity; and interpolating output testing data thatfalls outside of the measured non-linearity based on the boundingpoints.
 42. The system of claim 36, wherein the receiver is furtherconfigured to map the input operating point and output operating pointto a particular level to avoid signal saturation or fractional valuerepresentation overflow.
 43. The method of claim 2, wherein the step ofmeasuring the non-linearity of the TWTA comprises: generating adifference between an ideal signal and a received signal.
 44. The methodof claim 43, wherein generating a difference between an ideal signal anda received signal comprises: demodulating the received signal; decodingthe demodulated signal; generate the ideal signal; and subtracting theideal signal from the demodulated signal.
 45. The method of claim 43,wherein generating a difference between an ideal signal and a receivedsignal comprises: demodulating the received signal; decoding thedemodulated signal; generating the ideal signal with a carrier of thereceived signal; subtracting the ideal signal from the received signal.46. The method of claim 2, wherein the means for of measuring thenon-linearity of the TWTA comprises: generating a difference between anideal signal and a received signal.
 47. The method of claim 43, whereingenerating a difference between an ideal signal and a received signalcomprises; demodulating the received signal; decoding the demodulatedsignal; generate the ideal signal; and subtracting the ideal signal fromthe demodulated signal.
 48. The method of claim 43, wherein generating adifference between an ideal signal and a received signal comprises:demodulating the received signal; decoding the demodulated signal;generating the ideal signal with a carrier of the received signal;subtracting the ideal signal from the received signal.